Various methods and corresponding devices are used for the detection of gas for instance leaking from high-pressure gas installation pipelines in industrial plants or offshore installations. Thus, it is known to use catalytic point sensors or infrared point sensors where the sensor generates a signal after the sensor having been exposed to a certain minimum concentration of a gaseous agent for a given minimum of time. The concentration can be measured in LEL (Lower Explosive Level), which indicates when an explosive mixture is reached between the gas and the oxygen in normal air. Such sensors need thus to be in actual physical contact with the gaseous agent in order to generate a signal, which can for instance be used for triggering an alarm. According to a different method, so-called open path sensors are used, comprising an infrared (IR) transmitter and a corresponding receiver for receiving the IR signal transmitted by the transmitter and positioned at a certain distance from each other, i.e. with a certain signal propagation path there between. When a given concentration (LEL level) of a gaseous agent enters the propagation path between the transmitter and receiver, at least a portion of the IR signal is being absorbed by the gas and hence the portion actually received at the receiver decreases. Thus, the presence of a gaseous agent in the propagation path is detected by the receiver, which can for instance elicit an alarm signal.
By either of the above methods, it is possible not only to detect the presence of gas, but also to determine its concentration. It may, however, be desirable, either in combination with the above detection means or in itself, to provide a gas leak detector, which does not require the building-up of a given minimum concentration of gas before eliciting a detection signal, but which is capable of generating an instant alarm signal, for instance at the escape of gas through a leakage in a pipeline, etc.
A method and corresponding detector device can be based on the fact that the escape of gas at sufficiently high pressure through a leakage, for instance, in a pipeline, generates airborne acoustic sound waves of a very large bandwidth. The amplitude of the sound signal is partly related to the mass flow rate (leak rate) from the leakage. The leak rate of the gas from the leakage is measured in the unity Kg/Sec. The leak rate is mainly determined by the gas pressure immediately before (upstream of) the leakage and the size of the leak. Other factors, for instance the molecular weight of the gas and the gas temperature, have been found to have less influence on the sound signal generated by the leakage. Thus, the frequency spectrum of sound waves generated by this mechanism extends far into the ultrasonic region. The ultrasound portion of the signal generated by the leakage can thus be picked up by an ultrasound-receiving transducer, the positioning of which relative to the leakage will not be critical, which is of course important in installations comprising spatially widely distributed components that could potentially contain leakages. A very important advantage of this type of detector is that it does not require being in physical contact with the gas or that the gas be present in a predetermined propagation path. The correct functioning of a detector based on this principle is therefore less affected by wind carrying the gas away from the detector or by the gradual dilution of the gas as the gas spreads over a larger area.
Gas leak detectors of the above kind are known within the art and are known to function satisfactorily at distances between a leakage and a detector of up to approximately 15 meters.
A system for detection of, for instance, leakages in pipelines, etc. of the above kind is disclosed in EP 0 315 199 where the system comprises detector devices provided with directional transducers for detecting ultrasonic vibrations generated by a leakage in the vicinity of the transducer. According to this document, detector devices are provided throughout the installation at critical locations points, valves etc.) where leakages could be expected to occur. Thus, this system is specifically designed for use in installations where a certain a priori knowledge of the location of possible leakages is present and where consequently detector transducers with a narrow directional characteristic pointing directly towards the critical location can advantageously be used.
When such gas leak detectors are installed at a site, such as an off-shore installation remote from a central alarm system, it is important continuously to ascertain that the detector is functioning reliably, i.e. that the absence of a signal from the detector indicating the presence of a gas leak is actually due to the fact that no such leak is present in the installation and is not caused by malfunctioning of the detector device itself. It is thus important that a detector of this type be provided with means for performing a self-test procedure, the self-test comprising all components of the detector, i.e. not merely the electronic circuits of the detector, but also the detector transducer and optional wind screens or other protective devices encapsulating the transducer. Advantageously, the test system and procedure should provide for a simple implementation hereof.